Chelmsford Crown Court heard that the explosion caused a fire at the substation.
John Higgins, 59, from Colchester died at the scene despite the arrival of Essex Fire and Rescue within minutes of the alarm being raised by staff from nearby Anglia Ruskin University.
The incident also blacked out a large part of Chelmsford, including Broomfield Hospital.
An investigation by the Health and Safety Executive found that UK Power Networks had failed to properly assess work with tap changers and to devise procedures for the work. It had also failed to adequately train employees for carrying out this task.
UK Power Networks has since introduced revised procedures to safeguard staff.
UK Power Networks (Operations) Limited was fined £275,000 with £145,000 in prosecution costs after pleading guilty to safety breaches.
After sentencing HSE Inspector Steven Gill said: “John Higgins lost his life in tragic circumstances that could have been avoided had this activity had been properly assessed and managed by UK Power Networks.
“His death illustrates how dangerous work on or near electrical distribution networks can be, and how imperative it is that employers – large or small – ensure that all activities involving high voltage electrical equipment are properly assessed and that safe systems of work in place.
“There is no room for error or complacency when working with high voltage equipment.”